The present invention relates to a toner composition for electrophotography, and more particularly to an electrophotographic toner composition having excellent fixing property at a low temperature, offset resistance, blocking resistance and the like which contains a specific polyester resin as a binder.
Various electrophotographic methods are hitherto known. There is generally known a method in which a latent electrostatic image is formed on a photosensitive body utilizing a photoconductive material by various means and is then developed with a toner, and after transferring the toner image to a support such as paper if necessary, the toner image is fixed by heating, applying a pressure or using a solvent.
In recent years, a high speed fixing property is required for the toner in order to increase the efficiency of the copying operation. Attempts such as using a thermoplastic resin having a lower softening point and being easily heat fusible as a toner binder to be used in a conventional oven-heat fixing method have been made to provide a toner composition satisfying the requirement. However, mere lowering of the softening point of a resin binder has the defect of causing agglomeration of toner particles, namely blocking phenomenon, during strage or use. Accordingly, as one system for high speed fixing, there has been utilized a fixing method using a heat roller having a good heat conductivity.
Since the heat roller system is higher in heat efficiency than the heat fixing system, a toner can be fixed in a shortened period of time. However, sticking of the toner to the heat roller, namely the so-called offset phenemenon, occurs, since the toner is directly brought into contact with the heat roller. Accordingly, it has been desired to develop a resin capable of solving this problem. A styrene-acrylate copolymer and a styrene-butadiene copolymer are usually known as a resin binder for a toner. These vinyl copolymers are in general poor in offset resistance and fixing property at a low temperature.
One of other resin binders is a bisphenol type epoxy resin. The bisphenol type epoxy resin provides a toner having superior fixing property at a low temperature because of having a lower molecular weight as compared with the vinyl copolymer binder. But, it has a tendency to lowering the offset resistance, because the melt viscosity of the resin is low.
Recently, various polyester resins are proposed as a resin binder used in a toner which is excellent in both offset resistance and fixing property at a low temperature. As disclosed in, e.g., Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 109825/1982, a polyester resin produced by employing an etherificated diphenol as a main component of alcohol and modifying an obtained polyester resin with dicarboxylic acids substituted by a long chain alkyl group has been investigated from the viewpoint of the blocking resistance and offset resistance.
However, according to the inventors' researches, since the dicarboxylic acids substituted by a long chain alkyl group which are used as a modifying component remarkably lower the softening point of the obtained polyester resin, the object of the technique described in the above-mentioned Tokkyo Kokai No. 109825/1982 that an etherificated diphenol is employed as a main component of alcohol in order to maintain the softening point of the polyester resin to be high can not be sufficiently accomplished. Therefore, it has a defect that it is difficult to maintain the balance of the blocking resistance and the fixing property.
Further, as disclosed in Tokkyo Kokai Nos. 29255/1984 and 29256/1984, amorphous polyester resins prepared from etherficated diphenols as a main component of alcohol have been investigated. According to the above-mentioned Tokkyo Kokai Nos. 29255/1984 and 29256/1984, the etherificated diphenols are used as a main component of alcohol in order to maintain the softening point of the polyester resin to be high, and thereby the blocking resistance and the offset resistance of the obtained toner are improved. However, according to the inventors' researches, they should be improved on the point of fixing property at a low temperature.
Accordingly, the present inventors have been examined to develop a polyester resin maintaining the softening point to be high and having well-balanced blocking resistance and fixing property at a low temperature.
It is an object of the present invention to provide a novel electrophotographic toner composition which can solve various problems that conventional techniques can not solve, that is, an electrophotographic toner composition which satisfies well-balanced offset resistance, blocking resistance and fixing property at a low temperature which are required for an electrophotographic toner composition containing a polyester resin as a binder at a time and, in particular, which is suitably used in high speed machines has not yet been obtained.
In general, it is supposed that the improvement of fixing property at a low temperature of the electrophotographic toner composition will be accomplished by lowering the softening point or molecular weight of the binder. Therefore, the inventors have produced various modified polyester resins by introducing a monocarboxylic acid component into polyester resins described in Tokkyo Kokai Nos. 29255/1984 and 29256/1984 and estimated the properties of the toner prepared from the modified polyester resin. However, the toner has not yet satisfied the above properties at the same time.
The present inventors have found that all of the above-mentioned defects can be solved when a polyester resin containing a specific alcohol component, that is, a polyester resin prepared by utilizing a rosin glycidyl ester as one of alcohol components, is employed as a binder. Further, they have found the fact that the fixing property at a low temperature is remarkably improved in case a monocarboxylic acid is employed as one of the acid components of the polyester resin.
In accordance with the above-mentioned their researches, they have eventually accomplished the present invention.